The present invention relates to a method and apparatus for improved separation of particulate solids from a solids suspension in a flowing gas stream. More particularly, the present invention relates to an improved cyclone separator device for enhanced removal of a suspension of particulate solids from a flowing gas stream.
The present New Source Performance Standards (NSPS) for utility boilers limit particulate emissions to 0.03 pound of total suspended particles per million BTU of heat input. To meet NSPS the particulate removal efficiency of a control device must be 99.0 to 99.9%, depending upon the heating value of the coal. The NSPS have been established because fine particulates in the size range of 0.5 to 10 microns can be inhaled into the human respiratory tract where they may remain deposited in the lower respiratory system for quite some time. Accordingly, they are referred to as respirable particles. They include solid particles and aerosols originating from condensable gases or vapors produced during coal volatilization and combustion. They may contain sulphates, nitrates, trace metals and, sometimes, organic compounds that are suspected of being irritants, carcinogens, mutagens, or toxins.
In general, four types of apparatus are used to remove such particulates from the flue gas. They are electrostatic precipitators, fabric filters, venturi scrubbers, and multiple cyclones.
Electrostatic precipitators can remove particulates from a gas stream down to a size of about one micron, at best, and at an efficiency of from 95 to 99%. They have a low pressure drop, but they are more expensive to install initially. However, they only incur nominal operating and maintenance costs. One advantage is that they are capable of operating at high temperatures in the range of from about 300.degree. F. to 450.degree. F. They require a large size facility, however, and they are sensitive to dust resistivity and flue gas characteristics, such as the metal oxide and sulphur content. Dust resistivity relates to the ability of the particulate matter to hold an electrostatic charge. Moisture in the gas is a factor, as well as the sulphur content and the presence of metal oxides. Additionally, electrostatic precipitators require safeguards due to the high voltage environment.
Fabric filters are capable of removing particles which have a size of less than one micron at an efficiency which is greater than 99%. Although they are less expensive to install initially, they are more expensive to operate due to the replacement of bags. The advantage of the fabric filters is that not only do they collect submicron particles, but they can provide a continuing reaction surface if coupled with a dry sulphur dioxide scrubber. In such an operation the bags will contain lime, which is reactive to the sulphur dioxide, and as the gas passes through the fabric of the bag, the sulphur dioxide reacts with the lime for chemical removal. A disadvantage of fabric filters is that they are incapable of operating at a high temperature, since high temperatures cause degradation of the bags. Additionally, they have a very high pressure drop. Moreover, the bags are susceptible to chemical attack and moist dust. Regarding moist dust, water vapor in the flue gas condenses in the bag filter and the resulting wet dust causes the bags to become gummy.
Venturi scrubbers are capable of removing particulates from the gas stream down to a size of about 0.5 micron and at a separation efficiency approaching 99%. They have a high pressure drop. They also have a high power cost, but the installation requires little floor space. They are capable of collecting very fine particulates and require very little maintenance. Additionally, they can remove some sulphur dioxide and they are insensitive to the type of ash and the condition of the flue gas. However, they cause secondary pollution by the waste water, and they additionally have corrosion problems. Accordingly, venturi scrubbers are currently losing favor in the utilities industry.
Multiple cyclones are capable of removing particles having a size down to about five microns at an efficiency of only 50 to 90%. Under ideal conditions, a maximum efficiency of 95% may be experienced. The pressure drop is generally low to moderate, but in some installations there can be a high pressure drop. They require a relatively low capital investment, they are nearly maintenance free, and they have a simplicity of design. However, efficiency is variable because they are susceptible to fluctuations in flue gas flow rate, temperature level, and dust loading. Moreover, they are unable to meet the regulations of the Environmental Protection Agency.
With this then being the state of the art., it is an object of the present invention to provide an improved method and apparatus for the removal of a suspension of particulates from a flowing gas stream.
It is another object of the present invention to provide an improved method and apparatus for the enhanced removal of particulates from a gas stream at high separation efficiency.
It is a further object of the present invention to provide an improved method and apparatus for enhanced removal of suspended particulates from a flowing gas stream without requiring excessive capital, operating costs, or maintenance expense.
These and other objects of the present invention, as well as the advantages thereof, will become more clear from the description which follows.